The FBI will release video showing suspects in the Boston Marathon bombing at a momentary news conference in Boston, Fox News has learned.

The video is believed to be of the crowd around the finish line of Monday’s race where two bombs exploded, killing three and injuring 176. Sources said the FBI did not release the images earlier because they still had “groundwork” to do. It was not known if the suspects have been identified.

Earlier, Department of Homeland Security Secretary Janet Napolitano said investigators were seeking individuals seen in footage or photos. Napolitano stopped short of calling the individuals suspects or describing them or their demeanor in her remarks, made to the House Homeland Security Committee on Thursday. Napolitano also said it remains unclear whether the bombs that exploded near the finish line of the storied race, killing three and injuring 176, were the work of foreign or domestic terrorists. She said the investigation is continuing “apace.”

“We have been collecting video from a variety of sources, as you might imagine, at the finish line of the Boston Marathon,” Napolitano said. “There’s lots and lots of video. There is some video that has raised the question of those that the FBI would like to speak with. I wouldn’t characterize them as suspects under the technical term. But we need the public’s help in locating these individuals.”

FBI investigators have been scouring video and still images taken at Monday’s race. Authorities believe at least one of the bombs was a sealed pressure cooker laden with explosives and shrapnel, and may have been concealed in a backpack. Investigators are believed to be weighing the advantages of releasing the photos and enlisting the public’s aid in finding possible suspects and doing anything that might jeopardize the investigation. Amateur sleuths around the world have been examining widely circulated photos from the crowd, isolating on people with backpacks, but officials have warned against such speculation.

A mangled pressure cooker lid found atop a nearby building is believed to have been part of one bomb, and it and other pieces were being analyzed at an FBI lab. A battery and several pieces of shrapnel were also recovered and undergoing analysis. Fox News learned that the circuit board suspected of being used to detonate at least one of the bombs has been recovered, and that FBI investigators were also analyzing cellphone tower records to identify positive hits for signs of calls that may have been placed to trigger both explosions remotely.

Authorities are also interested in a battery believed to be used in one of the bombs, telling Fox News it was likely purchased with a remote control toy and then extracted the battery to use in the bomb. That could potentially make it easier to zero in on a suspect.

According to a FBI and Department of Homeland Security bulletin, the deadly shrapnel that caused the deaths — including of an 8-year-old boy, and critical injuries to 17 — included nails, BBs and ball bearings. The other device “was also housed in a metal container, but investigators could not say if that was also a pressure cooker.

An investigative source also told Fox News that there is a “significant social media footprint” on the bombings that is providing new leads to investigators. More than 30,000 social media messages were collected within a one-mile radius of the finish line in the 48-hour period surrounding the explosions – with “Twitter and Facebook lighting up” after the attack. The social media generated what are called link analysis charts – which showed “the relationships between social media messages that met investigative criteria.” Investigators are especially interested in messages that seemed “out of place or coded,” sources told Fox News.

Monday’s horror unfolded just before 3 p.m., shattering a festive atmosphere several hours after the legendary race began on the city’s 238th annual Patriots’ Day. In the aftermath, officials found bomb remnants, shrapnel and shredded backpacks believed to have concealed the deadly payloads.

Richard DesLauriers, FBI agent in charge in Boston, confirmed at a press conference investigators had found pieces of black nylon from a bag or backpack and fragments of BBs and nails, possibly contained in a pressure cooker. He said the items were sent to the FBI laboratory at Quantico, Va., for analysis.

Investigators are also examining if the bombs could have been assembled near the scene of the explosions, The Wall Street Journal reports, quoting a law enforcement official. The official says this possibility is being considered because transporting improvised devices over a significant distance could trigger a premature detonation.

Scores of victims remained in hospitals, but the death toll has not risen since Monday.

Boston Medical Center held a press conference Thursday morning and said it saw a total of 23 patients. Doctors there performed 16 operations within the first several hours and, on Wednesday, performed 13 reoperations. There were four patients released since the explosion and one remains in critical condition, the doctors said. Ten patients are in serious condition.

Tufts Medical Center said it is treating five patients, none of whom are listed in critical condition. In total, nine patients have been discharged.

The doctors from Boston Medical Center credited some recent advances to dealing with trauma from techniques used in Iraq and Afghanistan. For one, doctors and first responders used component therapy instead of a lot of IV fluids. Component therapy can be used to promote blood clotting.

Anyone with information on the bombings is being urged to call Boston authorities at 1-800-494-TIPS.